The present invention relates to gas turbines and, more particularly, to an improved stator vane assembly using ceramic vanes.
Significant improvements can be made in the efficiency and performance of gas turbines by the use of ceramic elements to permit operation at higher temperatures or with less cooling. In particular, the use of uncooled ceramic stator vanes, especially in the first row of stationary vanes, makes possible a very substantial improvement in efficiency. Because of the mechanical properties of ceramic materials, it has been found that the most desirable construction for such a stator vane assembly involves the use of three-piece vane assemblies in which each airfoil vane is supported by a separate end cap at each end of the vane, as disclosed in a copending application of R. J. Schaller et al, Ser. No. 387,069, filed Aug. 9, 1973, now U.S. Pat. No. 3,857,649, and assigned to the Assignee of the present invention.
In the design of such a vane assembly, the junction between the airfoil vane and each of the end caps associated with it is critical. The junction must provide sufficient freedom for the vane to move relative to the end cap as necessary, and the design must be such that the junction is capable of supporting the forces applied to the vane which include not only the radial compression force for retaining the vane in position but also the forces due to the gas pressure on the vane as well as those due to thermal expansion and contraction. The junction must also maintain accurate vane-to-vane alignment in the complete assembly, and should prevent thermal ratcheting of the vane with respect to the end cap which could cause the vane to move out of position. The steady-state and transient stress concentrations, particularly in the end cap, must be minimized because of the sensitivity of ceramic materials to stress concentrations. A successful design must meet all these requirements, which precludes any simple support of the vane on the end caps.